feat: retro blog engine with swappable OS/console themes

Next.js 16 (App Router, TS) + SQLite (better-sqlite3) + marked.

Core: a shared semantic HTML skeleton (rb- classes in Shell.tsx) that
each theme reskins via a scoped [data-theme="..."] CSS file. Theme is
persisted in a cookie, resolved server-side in the root layout, and
swapped live by the client switcher (no reload, no FOUC).

- DB auto-migrates and seeds posts on first run (data/blog.db, gitignored)
- Pages: post list, post detail (markdown), about
- Themes shipped: Windows XP (Luna), Windows 9x, PlayStation 2
- Adding a skin = registry entry + one scoped CSS file

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/vercel.svg

@@ -0,0 +1 @@
+<svg fill="none" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1155 1000"><path d="m577.3 0 577.4 1000H0z" fill="#fff"/></svg>